1. Field of the Invention
The present invention relates to a composite material for a positive temperature coefficient thermistor (referred to PTC thermistor hereinafter), ceramics for use in the PTC thermistor and a method for manufacturing the PTC thermistor.
2. Description of the Related Art
Related art concerning the present invention are disclosed in Japanese laid-open Patent Application Nos. 3-88770 and No. 3-54165.
The former application discloses a barium titanate based semiconductor ceramic composition for use in the PTC thermistor containing, as principal components, 45 to 85 molar percentage (mol %) of BaTiO3, 1 to 20 mol % of PbTiO3, 1 to 20 mol % of SrTiO3 and 5 to 20 mol % of CaTiO3 as well as 0.1 to 0.3 mol % of a semiconductor forming agent, 0.006 to 0.025 mol % of Mn and 0.1 to 1 mol % of SiO2 as additives, wherein BaTiO3, PbTiO3, SrTiO3 and CaTiO3 as principal components are formed by a citric acid method.
The application describes that characteristic values such as a resistivity at room temperature of 8 xcexa9xc2x7cm or less (4 to 8 xcexa9xc2x7cm), a temperature resistance coefficient xcex1 of 9%/xc2x0 C. or more and an static withstanding voltage of 60 V/mm or more can be obtained with respect to the ceramic composition. The laid-open text also shows examples using La, Sb and Nb as semiconductor forming agents.
The latter laid-open text discloses a barium titanate based semiconductor ceramic composition for use in the PTC thermistor containing, as principal components, 45 to 87 mol % of BaTiO3, 3 to 20 mol % of PbTiO3, 5 to 20 mol % of SrTiO3 and 5 to 15 mol % of CaTiO3 as well as 0.2 to 0.5 mol % of a semiconductor forming agent, 0.02 to 0.08 mol % of Mn and 0 to 0.45 mol % of SiO2 as additives, wherein BaTiO3, PbTiO3, SrTiO3 and CaTiO3 as principal components are formed by a liquid phase method.
The publication shows that characteristic values such as a resistivity at room temperature of 3 to 10 xcexa9xc2x7cm and an static withstanding voltage of 10 to 200 V/mm can be obtained with respect to the ceramic composition. It also shows examples using Sb, Y and La as semiconductor forming agents.
La, Sb, Nb and Y are used as a semiconductor forming agent in the examples disclosed in the foregoing two patent publications. However, these semiconductor forming agents involve problems, as shown in the comparative examples hereinafter. It is a problem that the resistance is largely dispersed when La, Sb or Nb are used for the semiconductor forming agent, although the resistivity at room temperature becomes low. The resistivity at room temperature can not be lowered when Y is used for the semiconductor forming agent.
The present invention provides a composite material for the PTC thermistor as well as a ceramic for use in the PTC thermistor obtained by firing the composite material, and a method for manufacturing the same.
In a first aspect, the present invention provides, in short, a composite material for the PTC thermistor using Sm as a semiconductor forming agent or, in more detail, a composite material for a PTC thermistor containing as principal components about 30 to 97 molar percentage (mol %) of BaTiO3, about 1 to 50 mol % of PbTiO3, about 1 to 30 mol % of SrTiO3 and about 1 to 25 mol % of CaTiO3 (wherein the total content of them accounts for 100 mol %), and containing as additives about 0.1 to 0.3 mole of Sm in terms of elemental Sm in a compound containing Sm, about 0.01 to 0.03 mole of Mn in terms of elemental Mn in a compound containing Mn and 0 to about 2.0 mole of Si in terms of elemental Si in a compound containing Si, relative to 100 mole of the principal components.
The present invention also provides a ceramic for use in the PTC thermistor obtained by firing the composite material as described above. In a second aspect, the present invention provides a ceramic for use in a PTC thermistor containing as principal components about 30 to 97 molar percentage (mol %) of BaTiO3, about 1 to 50 mol % of PbTiO3, about 1 to 30 mol % of SrTiO3 and about 1 to 25 mol % of CaTiO3 (wherein the total content of them accounts for 100 mol %), and containing as additives about 0.1 to 0.3 mole of Sm calculated as Sm in samarium oxide, about 0.01 to 0.03 mole of Mn in terms of Mn in manganese oxide and 0 to about 2.0 mole of Si in terms of Si in silicon oxide, relative to 100 mole of the principal components.
The present invention also provides a ceramic for use in the PTC thermistor obtained by firing the composite material for the PTC thermistor according to the first aspect in a neutral atmosphere followed by heat-treatment in an oxidative atmosphere.
The present invention also provides a ceramic for use in the PTC thermistor obtained by firing the composite material for the PTC thermistor according to the first aspect in a reducing atmosphere followed by heat-treatment in an oxidative atmosphere.
The present invention also provides a method for manufacturing a ceramic for use in the PTC thermistor.
In one aspect, the present invention provides a method for manufacturing the ceramic for use in the PTC thermistor with the step of firing the composite material for the PTC thermistor in a neutral atmosphere followed by heat-treating in an oxidative atmosphere.
In another aspect, the present invention provides a method for manufacturing the ceramic for use in the PTC thermistor with the step of firing the composite material for the PTC thermistor in a reducing atmosphere followed by heat-treatment in an oxidative atmosphere.
A nitrogen atmosphere may be used for the neutral atmosphere while air containing about 20% of oxygen or a high oxygen containing atmosphere containing about 100% of oxygen may be used for the oxidative atmosphere. An atmosphere containing 1% of hydrogen and 99% of nitrogen may be used for the reducing atmosphere.